1. Field of the Invention
This invention relates to a fluid pressure control method of an anti-skid control apparatus for a vehicle by which a brake fluid pressure of a wheel cylinder can be controlled in accordance with a skid condition of a wheel.
2. Description of the Prior Art
Various fluid pressure control methods have been developed for anti-skid control apparatus. In one fluid pressure control method, wheel speed sensors are arranged for four wheels, respectively. A fluid pressure control valve is provided for front and rear wheels diagonally connected to each other. Fluid pressures of the front and rear wheels are decreased and maintained at constant by "Select-Low" control.
In another fluid pressure control method, there is provided pressure-selecting means (so-called "Select-Low Valve") between both front wheels and both rear wheels. The fluid pressure control valves are controlled by "Select-Low" control of the front and rear wheels on the same side of the road.
The above methods have the respective effects, and assure the driver of safety. However, it is required for more accurate anti-skid control that wheel speed sensors and fluid pressure control valves be provided for the respective wheels. Such an anti-skid control apparatus is a so called "four-channel, four-sensor type". A control unit judges the respective skid conditions of the wheels, and generates control signals representing the skid conditions of the respective wheels. There are two kinds of control methods for the four-channel, four-sensor type control unit. One is a "Select-Low" control of the two rear wheels. Another is "Independent control" of the two rear wheels. In the "Select-Low", when a control signal, namely a decreasing control signal or a holding control signal is generated from any one of the two rear wheels, both of the rear wheels are controlled on the basis of the generated control signal, although no control signal is generated from another rear wheel.
FIG. 1 shows a logic diagram of the "Select-Low". "AVFL", "AVRR", "AVRL", "AVFR" on the left of FIG. 1 represent AV signals (brake decreasing control signals-"AV" means "brake decreasing") of left front wheel (FL), right rear wheel (RR), left rear wheel (RL) and right front wheel (FR), respectively. And "EVFL", "EVRR", "EVRL", "EVFR" on the left of FIG. 1 represent EV signals (brake holding signals-"EV" means "brake holding") of left front wheel (FL), right rear wheel (RR), left rear wheel (RL) and right front wheel (FR), respectively. "(AVFL)", "(AVFR)" on the right of FIG. 1 represent drive signals (AV) for driving change-over valves to decrease the brake fluid pressures of the front left and rear wheels, respectively, and "(EVFL)", "(EVFR)" on the right of FIG. 1 represent drive signals (EV) for driving change-over valves to hold the brake fluid pressure of the front left and rear wheels. The drive signals (AV) and (EV) are formed in drive circuits L.sub.1 and L.sub.2. The above control signals AVRR, EVRR, AVRL and EVRL are supplied to a Select-Low circuit Q. The circuit Q includes a drive circuit. It generates a drive signal (AV-RR&RL) for driving change-over valves to decrease the brake fluid pressures of the rear left and rear wheels or another drive signal (EV-RR&RL) for driving the change-over valves to hold the brake fluid pressures of the rear left and rear wheels. The change-over valve for the front wheel FL or FR is controlled independent of the skid conditions of another front wheel FR or FL and the rear wheels RL and RR. However, the rear wheels are controlled in the "Select-Low", and both of the rear wheels are controlled in the same manner.
However, when the vehicle is running on a split road in which the coefficients (.mu.) of the sides are remarkably different from each other, or when the vehicle is cornering, both of the rear wheels are controlled in accordance with the one rear wheel on the low-.mu. side or the inside during the cornering, in the "Select-Low" method. A larger braking force to be applied, cannot be applied to the other rear wheel on the high-.mu. side or the outside, although they are not yet at a locked state. Indeed, the security or safety can be obtained, but the braking distance is lengthened. That is a disadvantage.
As above described, in the "Select-Low" manner, both of the fluid pressures of the rear wheels are controlled in accordance with the skid condition of the one rear wheel on the low-.mu. side. Accordingly, when the difference between the coefficients of the road sides is remarkably large, a braking force of the other rear wheel on the high-.mu. side becomes very small in contrast to a braking force to be applied. When a load applied to the rear wheels is larger, the above tendency is more remarkable.
On the other hand, when the fluid pressures of the left and right rear wheels are controlled independently of each other, braking forces to the rear wheels can be larger than in the "Select-low" method. However, the stability of the vehicle is deteriorated. There is some difference between the coefficients of the road sides. When the difference is large, the deterioration of the steering stability is remarkable. The reason is that a braking force to the one rear wheel on the low-.mu. side is small, and that a braking force to the other rear wheel on the high-.mu. side is large. In that case, a rotational force is imparted around the center of gravity of the vehicle. It is a so called "Spinning force". Over-steering tendency becomes remarkable. When the vehicle is cornering, a similar tendency occurs. The spinning of the vehicle is very dangerous. Further, when a braking force is larger, the wheel is apt to lock. The side force is decreased. The vehicle is apt to slip sidewards. As above described, in the "Dependent Control" of the rear wheels, the side force of the rear wheel is decreased and so it is difficult to obtain the runnning stability of the vehicle.